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7 Common Lisp Condition System

One of Lisp’s great features is its condition system. It serves a similar purpose to the exception handling systems in Java, Python, and C++ but is more flexible. Its flexibility extends beyond error handling—conditions are more general than exceptions in that a condition can represent any occurrence during a program’s execution that may be of interest to code at different levels on the call stack.

The condition system is more flexible than exception systems because instead of providing a two-part division between the code that signals an error and the code that handles it, the condition system splits the responsibilities into three parts:

1. signaling a condition,
2. handling it, and
3. restarting.

In this chapter, I’ll describe how you could use conditions in part of a hypothetical application for analyzing log files. You’ll see how you could use the condition system:U

• to allow a low-level function to detect a problem while parsing a log file and signal an error,
• to allow mid-level code to provide several possible ways of recovering from such an error, and
• to allow code at the highest level of the application to define a policy for choosing which recovery strategy to use.

• Terminology
• Common Lisp Conditions